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should be removed with a pipette and measured, the value given by the
subsequent determination being suitably increased.

In a glass or copper distillation flask, a weighed quantity of 100 grams
of the well-mixed sample is distilled with 50 c.c. of benzene (90% and 50%)
through a condenser. The distillation is carried up to 190° in about half
an hour, the distillate being collected in a graduated cylinder and the volume
of the aqueous layer read off. Industrially, the water is determined directly
during the distillation test and is collected with the light oils, from which
it separates on standing, so that it may be easily measured.

2.  Determination of the Specific Gravity.—The tar is first com-
pletely freed from water.   To this end it is left for 24 hours in a closed vessel
in a bath of water heated to a temperature not higher than 50°, being shaken
from tune to time to facilitate the rising  of  the drops of water and air
bubbles.   When the layer of water is thoroughly separated, it is decanted
or siphoned off and the specific gravity of the residual tar determined at
15° C.   With a fairly mobile tar, an ordinary densimeter or picnometer is
used, but with very dense tar either a picnometer for solids, with a wide
mouth and a ground stopper surmounted by a tube with a mark on it, or
an ordinary weighing bottle with a rill in the stopper x may be used.

3.  Determination   of  the  Free   Carbon.—According   to   Kohler's
method,2 10 grams of the tar are boiled with 25 c.c. of acetic acid and 25
c.c. of toluene in a conical flask with a reflux apparatus and the hot liquid
filtered through two filter-papers reduced to equal weight and placed one
inside the other.   The residue on the filter is washed with hot toluene until
the latter passes through colourless, the two filters being then separated
and dried at about 120° until of constant weight.    The difference in weight
between the two filters gives the free carbon.

From the content of free carbon (c) thus obtained, the yield of a tar
in pitch of a definite hardness may be determined—knowing the proportion,

k, of free carbon in the pitch—by the formula x =-------


Assuming that, for a good pitch of medium hardness, k is 28%, a tar con-
taining c% of free carbon will give x = (looc -f- 28) % of such pitch.

4.  Fractional  Distillation.—Fractional   distillation   of  tar  presents
difficulties on account of the bumping, which is due mainly to the presence
of water.    It is, therefore, necessary first to dehydrate the tar as completely
as possible in the manner indicated above, then to distil from a flask not
more than half full and to heat with great care until all the residual water
is eliminated.   The apparatus used for the distillation of mineral oils may
be employed (see Chapter VIII).    It is also advantageous to pass through
the boiling liquid a gentle current of air by means of a capillary tube dipping
into the liquid, the boiling being thus rendered more even.    During the
initial stages of the distillation use is made of a condenser, which is removed
when the distillate tends to solidifylin the tube.   As regards the limits of

* KJ                                                                  ^

temperature, four successive fractions are usually collected :

1  See Lunge :  Coal Tar and Ammonia, 1916, Part I, p. 520.

2  Dingler's Polyt. Journ., 1888, 270, p. 233.0